You can buy lifetime service
Tivo sells this way to make the product a cheap purchase upfront. Its like AT&T selling iPhones for $199, but then requiring a two year commitment. You can't complain about having to pay a monthly fee when the regular price of a Tivo is $99 and they occasionally have offers selling them at half price or even for free. If you got your Tivo for free would you say that the monthly fee is a bad deal? Well, maybe after you'd had it for the better part of a decade and had paid over $1000 in monthly fees, but you have a choice to avoid the monthly fee...
If you wish, you can buy a Tivo for $99 and then buy lifetime service for $399. Tivo.com says $499, but it is easy for even first time owners to get the discount for existing owners to $399 (just google it) I bought mine in April 2010 for $199 but the discounted lifetime service at that time was only $299 so same price. You buy that and you get Tivo service (guide, software upgrades, etc.) as long as it functions, and it transfers with the Tivo if you sell it.
I'm sure there are some DVRs I could buy for less, and certainly I could build one for less if I decide my time has no value. But the software probably doesn't work as well and is more likely to have compatibility issues with the cablecard (I know a couple people who tried to save money by building their own, it took them a lot of work to get everything finally working right)
It isn't the best option for everyone, but for me at least my cable company charges $12.95 a month for a dual tuner HD DVR that's theoretically equivalent to my Tivo. So around June next year it'll have cost me less to buy the Tivo than use the cable company's DVR. But if you consider resale value, which Tivos with lifetime service maintain very well, I was already ahead of using the cable company's DVR in less than a year.
I'm sure at this point someone will want to whine that the lifetime service is only good for the lifetime of the unit, so if it dies after the warranty expires in a year I have to buy a whole new Tivo and lifetime service again. Yes, that's true. If that worries you, you buy an extended warranty. It is no different than what happens if you buy a TV or laptop that breaks after the warranty is over. Tivos are quite reliable, and I personally feel extended warranties are a bad buy for anyone who can afford to take the chance. Well, except for Applecare on an iPhone if you have a fumble fingered girlfriend :)